Light-sensitive electric device



Jan. 1, 1929. v

J. L. BAIRD LIGHT SENSITIVE ELECTRIC DEVICE Filed Ma 8, 1928 IIYYFITM IIS A w Patented Jan. ;1,- 1929.

UNITED STATES- PATENT oFFlCEa JOHN Loam BAIBD, or LONDON, ENGLAND,-assIeNon r 'rnmivrsroN Lnrrrnn, or

LONDON, ENG AND;

men'r-saNsrrrvn Application filed Kay 8, 1928, Serial No.

I This invention is for improvements in or relating to light-sensitiveelectric devices of the type in which light-sensitive materials such asselenium, thallium sulphide, carbon and so forth are used, thesebeingsubstances which chan e their electrical resistance when expose toradiant energy; the object of the invention is to provide a constructionwhereby the time-lag of the response of the device is reduced oreliminated, and in-some instances the device is rendered more sensitive.

The invention accordingly comprises a light-sensitive cell wherein thelight-sensitive substance is in the form of a thin film.

which provides the maximum active area with the minimum weight and heatcapacity, so that any response of the material due to change intemperature may, where suitable, be utilized in a'ddition'to thephotoelectric effect. I

The thin film aforesaid of the-light-sensitive substance, may, accordingto another feature of this invention, be made so thin that it cannotsupport itself, and in this case the film is disposed on an insulatingsupport which is itself of theminimum thickness necessary to provide themechanical support'for the film. With this arrangement, 80 the thin filmmay conveniently be'ohtained by sputtering it 'on the supportingelement. The electrical resistance of selenium and other light-sensitivesubstances varies with its temperature, and the temperature coefficientitself varies, being positive over some ranges of temperature andnegative over others: this variation is independent of the resistancevariation due to the action of light, The present invention therefore 40comprises further the combination with a light-sensitive cell, of meansfor controlling its temperature whereby it may be operated incircumstances in which the thermal effect is additive to the opticaleflect when radiation falls on the cell, and the sensitivity is therebyconsiderably increased. This invention also comprises the combinationwith a light-sensitive cell, its circuits and amplifying devices, ofmeans for controlling the temperature of some or all of these parts inorder that parasitic currents in the conductors may beaminimized bymaintaining them at a low temperature.

This invention also comprises a construc- 56 tion of light-sensitivecell in which a sub- A Bnrrrsn COMPANY.

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stantially closedchamber has a light-sensitive substance dis in the formof a thin coating onits inner surface, and preferably extending alloverthat surface: this chamber has an opening through which radlationcan enter 1t to impinge on the lightsensitive substance and be entirelyabsorbed thereby. I y

According-to another feature of this invention a light-sensitive cellmay be constituted by a light-sensitive fluid or liquid in the form of athin layer enclosed between two transparent plates or one opaque and.

one transparent plate.

In the accompanying drawings, which illustrate this inventiondiagrammatically F1gure 1 is a perspective view showing a thin film oflight-sensitive material mounted on a supporting element;

Flgure 2 shows an alternative construction in which the light-sensitiveelement is self-supporting;

Figure 3 is a sectional view showing diagrammatically the {cell adaptedfor temperature control; and

Figures 4 to 8 show diagrammatically various constructions oflight-sensitive cell.

In the construction illustrated in" Figure 1, a very thin sheet of micaor other insulating material 10' has a thin film of selenium sputteredon to it, for instance, by'means of an electric are, using the materialselenium as one electrode. Electrical contacts are provided at the twoends of the strip in any convenient manner, as for example by springclips 12.- The mica or other insulating support is made of the minimumthickness which will provide suflicient mechanical strength. andrigidity, and the film of selenium is so thin that it would not bemechanically self-supporting.

In the construction illustrated in Figure 2, a single large crystal ofselenium may have conductors attached to its opposite ends 13, 14, andthe intervening material is ground away as at'15 so as to, make it asthin as is consistent with the necessary 6 temperature.

strip is but small, the heat-capacity of the terminals is not of suchgreat importance.

Figure 3. illustrates diagrammatically a light-sensitive j cell in whichprovision is made for regulating the temperature. It comprises alight-sensitive element 16 which may for example be similar to thatillustrated in Figure 1, and it is enclosed in a double-walledchamberlT. A glass, quartz or other transparent window 18 is provided inthis chamber to permit the entry of the radiant energy which is to acton the. lightsensitive material. The space between the double walls maybe used for. circulating a cooling or heating fluid throughpipe-connections 19, or alternatively or additionally a suitableresistance element 20 may be provided for electrically heating thevessel. This construction provides a device in which the temperature ofthe light-sensitive g element 16 can be positively controlled so that itis operated at the most advantageous When light or other radiant energyis to be measured by means of a selenium or similar light-sensitivecell, it is advantageous that the whole of the energy falling on thelight-sensitive material should be absorbed thereby, and none lost byreflection or diffusion, as the sensitivity is thereby increased. 7

With this object in view the light-sensitive cell may be constructed asillustrated dia- 4 the whole of its interior with thelight-sensigrammatically in Figure 4, in which a sub-' stantially closedchamber 21 is coated over tive material. A small opening 22 is providedto admit the radiation and with this arrangement it is ensured that allthe energy entering the chamber is absorbed, because any energyreflected from the surface is subjected to repeated reflection until itis all absorbed.

In this arrangement, the current, which is varied by the variation inresistance of the light-sensitive material, is arranged to traverse thewhole of the material, the circuitbeing established by means of twoleading-1n wires 23, 24 which -are connected to two points on the innercoating spaced apart, for example diametrically opposite one another.

In the construction illustrated in Figure 5 the chamber 25 maybe made ofmetal or other conducting material coated on its'inner surface with alayer 26 of light-sensitive material. The. aperture 27 by which thelight enters the cell is closedby glass, quartz or other materialtransparent to the radiation which is-being used. In this case thecurrent 1s made to traverse the thickness of the layer or film 26 in aradial direction, one pole of the cell being constituted by the chamber25 to which a connection 37 is made, and the other by acentrally-disposed nium is used as the end wall of a vessesl 30.

One connection may be made directly to the outside surface of the plate29, and the vessel is filled with a conducting liquid in which aterminal 31 constitutes the other pole of the cell. The walls of thevessel and the liquid are of course: such as to be transparent to theradiation-which is being used. A modified construction of such a Cell'isillustrated in Figure 7, in which the selenium vplate32 constitutes apartition in a vessel separating two bodies of liquid which'constitutethe electrodes of the cell. The liquid on that side of the partition 32whichis not exposed to the radiationmay be any suitable conductingliquid, opaque to means 'for controlling their temperature duringworking in order that they may be caused to operate over a suitabletemperature range is which the maximum sensitivity is obtained andparasitic currents are reduced. Also, the whole or any desired part ofthe circuits associated with the cell, such for example, as theamplifying devices are preferably also provided with means formaintaining them at a low temperature for the purpose of avoiding orminimizing parasitic currents. The temperature control of the wholeapparatus may be effected in any desired manner, for example byenclosing the whole of the apparatus in a suitable chamber whereof thetemperature can be controlled.

I claim 1. A resistance unit variable by the action of light, comprisingan opaqueinsulating hollow body provided with an aperture therein forthe passage of light, and having a thinfilm of light sensitive material.deposited on the interior thereof. f

A resistance unit variable by the action: of light, comprising an opaqueinsulating hollow body provided with an a erture therein for the passageof light, and aving a thin film of light sensitive material deposited onthe interior thereof, and means for controlling the temperature of saidbody. 3. A resistance unit variable by the action of light, comprisingan opaque insulating hollow body provided with an aperture therein, andhaving 'a film of light sensitive material deposited on the interiorthereof, a transparent member forming aclosure for said aperture forpermitt-in passage of light to the interior of said body and forretaining conducting fluid therein, an electrode extending into saidconducting fluid, and anelectrical connection to said light sensitivefilm.

4. A resistance unit variable by the action of'light, comprising anopaque insulating hollow body provided with an aperture therein, andhaving a film'of light sensitive material deposited on the interiorthereof, a

transparent member forming a closure for said aperture for permittingpassage of light to the interior of said body and for retaining aconducting fluid therein, an electrode extending into said conductingfluid, a ter-- of light comprising an opaque hollow bodyof substantiallyspherical shape provided with'an aperture therein for the passage oflight, and having a film of light-sensitive material deposited on themajor portion of the interior thereof.

In testimony whereof I aflix mysignature.

' JOHN LOGIE BAIRD.

